Minimum ventricular pacing to break the repetitive AR-VS pattern

ABSTRACT

An implantable cardiac pacing device delivering minimum ventricular pacing and an associated method control intervals timed by the device during sensing and pacing. An atrial-only pacing mode is set in response to sensing intrinsic ventricular events in the ventricular chamber. A first post-ventricular atrial refractory period is set following each of a plurality of ventricular events sensed in the ventricular chamber, and atrial events each being sensed during first post-ventricular atrial refractory periods are detected. A second post-ventricular atrial refractory period is set in response to detecting the atrial events each being sensed during the first post-ventricular atrial refractory period.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.12/604,578, filed Oct. 23, 2009, now U.S. Pat. No. 8,024,042, entitled“IMPROVEMENT TO MINIMUM VENTRICULAR PACING TO BREAK THE AR-VS PATTERN”,incorporated herein by reference in it's entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to an implantable medical device and associated method fordelivering minimum ventricular pacing.

BACKGROUND

Naturally conducted or intrinsic ventricular depolarizations have beenrecognized as being preferable over ventricular pacing in general andpacing in the right ventricular apex in particular. In order to minimizeor greatly reduce ventricular pacing, protocols have been developedthat, in general, utilize an atrial based timing mode that promotesintrinsic conduction whenever possible. Illustrative protocols aredescribed, for example, in U.S. Pat. No. 7,218,965 (Casavant), U.S. Pat.No. 6,772,005 (Casavant), and U.S. Pat. No. 7,248,924 (Casavant), all ofwhich are incorporated herein by reference in their entireties.

In general, during a minimum ventricular pacing (MVP) protocol,atrial-only pacing is delivered unless intrinsic ventricular events arenot sensed. When a ventricular event is not sensed between twoconsecutive atrial events, a ventricular backup pacing pulse may bedelivered after the second atrial event to avoid another cardiac cycleof asystole. When a specified number of atrial-only pacing cycles occurwithout sensing a ventricular event, the pacing device switches to adual chamber pacing mode and delivers atrial and ventricular pacingpulses coordinated at a programmed atrial-ventricular (AV) delay.

Accurate sensing of intrinsic cardiac activity is important inmaintaining the appropriate response of the pacing device and properlyswitching pacing mode without undue delay when changes inatrial-ventricular conductivity occur. When an atrial event is sensed,an atrial refractory period (ARP) is set. The atrial tissue cannot bedepolarized again until after a physiological refractory period hasexpired and any earlier signals sensed by the pacing device during theatrial refractory period are likely to be far-field signals or othernon-atrial events. As such, any “events” sensed during the ARP may benoted by the pacing device but are not used in resetting pacing escapeinterval timers used to control the timing and delivery of pacingpulses. When a ventricular event is sensed, a post-ventricular atrialrefractory period (PVARP) is set. Any “events” sensed using atrialsensing electrodes during the PVARP may be noted but are also not usedfor resetting any escape interval timers used for controlling pacingintervals. The PVARP is useful in preventing far-field R-waves frombeing sensed as P-waves.

Premature ventricular contractions (PVCs) or undersensing of an atrialP-wave may cause two consecutive ventricular sense events with nointervening atrial sense event. In these situations a PVARP is setfollowing the second ventricular sense event, which may be an extendedPVARP. The next atrial event may occur during the PVARP resulting in apattern of atrial refractory sense (AR) events followed by ventricularsensed (VS) events, i.e., an AR-VS pattern. This AR-VS pattern canbecome sustained as each subsequent ventricular sense event is followedby a PVARP, during which the next intrinsic atrial event occurs. TheAR-VS pattern can cause a delay in the first back-up ventricular pacingpulse when AV conduction block returns and can cause user confusion whenthe AR-VS sensing pattern is observed. Pacing devices typically recordcounts of atrial and ventricular paced and sensed events to determineand report paced events as a percentage of the events counted. Thisreporting provides the clinician with a measure of the amount of pacingthat is occurring. AR-VS event intervals may not be counted in suchdevice diagnostics (since AR events are commonly ignored) resulting inthe percentage of pacing being over-reported. A need remains, therefore,for improvements in MVP methods that avoid the aforementionedconsequences of sustained AR-VS sensing patterns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an implantable medical device (IMD) in which monitoringand pacing methods described herein may be implemented.

FIG. 2 is a functional block diagram of the IMD shown in FIG. 1according to one embodiment.

FIG. 3 is a timeline depicting sensed atrial and ventricular events.

FIG. 4 is an EGM recording illustrating the delivery of a back-upventricular pacing pulse during an atrial based pacing mode during MVP.

FIG. 5 is a flow chart of a method for terminating an AR-VS sensingpattern during MVP.

FIG. 6 is a time line of a sustained AR-VS pattern terminated by settinga single, shortened PVARP.

DETAILED DESCRIPTION

In the following description, references are made to illustrativeembodiments. It is understood that other embodiments may be utilizedwithout departing from the scope of the invention. In some instances,for purposes of clarity, identical reference numbers may be used in thedrawings to identify similar elements. As used herein, the term “module”refers to an application specific integrated circuit (ASIC), anelectronic circuit, a processor (shared, dedicated, or group) and memorythat execute one or more software or firmware programs, a combinationallogic circuit, or other suitable components that provide the describedfunctionality.

As used herein, an atrial based pacing mode is a mode that is programmedto pace in the atria, but only to sense in the ventricles. True singlechamber atrial pacing would imply that only a single lead is present andventricular activity may not be sensed in the ventricle nor wouldventricular pacing be deliverable. In the present context an IMDoperating in an atrial based mode includes at least ventricular sensingcapabilities. Though not required, such a device would generally includeventricular pacing as well. However, in order to deliver ventricularpacing the device would switch pacing modes from the atrial only pacingmode to a dual chamber pacing mode, such as DDD.

FIG. 1 depicts an implantable medical device (IMD) 14 in whichmonitoring and pacing methods described herein may be implemented.Various embodiments of the invention may be implemented in numeroustypes of implantable medical devices capable of sensing cardiac signals,such as pacemakers, ECG monitors, and hemodynamic monitors. IMD 14 isprovided for sensing intrinsic heart activity and delivering cardiacstimulation pulses in the form of pacing, cardioversion ordefibrillation therapy, as appropriate, to one or more heart chambers.

IMD 14 is shown in communication with a patient's heart 10 by way ofthree leads 16, 32 and 52. The heart 10 is shown in a partially cut-awayview illustrating the upper heart chambers, the right atrium (RA) andleft atrium (LA), and the lower heart chambers, the right ventricle (RV)and left ventricle (LV), and the coronary sinus (CS) in the right atriumleading into the great cardiac vein 48, which branches to form inferiorcardiac veins. Leads 16, 32 and 52 connect IMD 14 with the RA, the RVand the LV, respectively. Each lead has at least one electricalconductor and pace/sense electrode. A remote indifferent can electrodeis formed as part of the outer surface of the IMD housing 20. Thepace/sense electrodes and the remote indifferent can electrode can beselectively employed to provide a number of unipolar and bipolarpace/sense electrode combinations for pacing and sensing functions.

RA lead 16 is passed through a vein into the RA chamber and may beattached at its distal end to the RA wall using an optional fixationmember 17. RA lead 16 is formed with a connector 13 fitting into aconnector bore of IMD connector block 12 for electrically coupling RAtip electrode 19 and RA ring electrode 21 to IMD circuitry housed withinhousing 20 via insulated conductors extending within lead body 15. RAtip electrode 19 and RA ring electrode 21 may be used in a bipolarfashion, or in a unipolar fashion with IMD housing 20, for achieving RAstimulation and sensing of RA EGM signals. RA lead 16 is optionallyprovided with a coil electrode 18 that may be used for delivering highvoltage cardioversion/defibrillation pulses to heart 10 in response tothe detection of tachycardia or fibrillation.

RV lead 32 is passed through the RA into the RV where its distal end,carrying RV tip electrode 40 and RV ring electrode 38 provided forstimulation in the RV and sensing of RV EGM signals, is fixed in placein the RV apex by a distal fixation member 41. RV lead 32 optionallycarries a high-voltage coil electrode 60 for use in cardioverting anddefibrillating heart 10. RV lead 32 is formed with a connector 34fitting into a corresponding connector bore of IMD connector block 12.Connector 34 is coupled to electrically insulated conductors within leadbody 36 and connected with distal tip electrode 40, ring electrode 38and coil electrode 60.

Coronary sinus lead 52 is passed through the RA, into the CS and furtherinto a cardiac vein 48 to extend the distal LV tip electrode 50 and ringelectrode 62 alongside the LV chamber to achieve LV stimulation andsensing of LV EGM signals. The LV CS lead 52 is coupled at the proximalend connector 54 into a bore of IMD connector block 12 to provideelectrical coupling of conductors extending from electrodes 50 and 62within lead body 56 to IMD internal circuitry. In some embodiments, LVCS lead 52 could bear a proximal LA pace/sense electrode 51 positionedalong CS lead body 56 such that it is disposed proximate the LA for usein stimulating the LA and/or sensing LA EGM signals.

In addition to the lead-mounted electrodes, IMD 14 may include one ormore subcutaneous cardiac sensing electrodes (not shown) formed asuninsulated portions of the IMD housing 20 or included in the connectorblock 12. While a particular IMD system with associated leads andelectrodes is illustrated in FIG. 1, numerous implantable cardiacmonitoring, pacemaker and IMD system configurations are possible, whichmay include one or more leads deployed in transvenous, subcutaneous, orepicardial locations. The lead and electrode arrangements will depend onthe particular application. Methods described herein may also beimplemented in a subcutaneous cardiac monitor, pacemaker or IMD systemin which electrodes are formed as a part of the device housing and/orcarried by subcutaneous leads.

IMD 14 is shown as a multi-chamber device capable of sensing andstimulation in three or all four heart chambers. It is understood thatIMD 14 may be modified to operate as a dual chamber device or a singlechamber device having dual chamber sensing capabilities. Theillustrative embodiments described herein generally relate to apacemaker or IMD having at least dual chamber (atrial and ventricular)pacing and sensing capabilities. It is contemplated that the methodsdescribed, however, may be adapted for use in a single chamber device byusing far-field sensing of ventricular events or using a hemodynamicsignal for identifying the timing of ventricular mechanical events.Pacing devices configured to operate in a MVP mode of operation aretypically provided as dual chamber pacemakers.

FIG. 2 is a functional block diagram of the IMD 14 shown in FIG. 1according to one embodiment. IMD 14 generally includes timing andcontrol circuitry 152 and an operating system that may employmicroprocessor 154 or a digital state machine for timing sensing andtherapy delivery functions in accordance with a programmed operatingmode. Microprocessor 154 and associated memory 156 are coupled to thevarious components of IMD 14 via a data/address bus 155. IMD 14 includestherapy delivery module 150 for delivering electrical stimulationtherapies, such as cardiac pacing therapies and arrhythmia therapiesincluding cardioversion/defibrillation shocks and anti-tachycardiapacing (ATP), under the control of timing and control 152. Therapydelivery module 150 is typically coupled to two or more electrodes 168via an optional switch matrix 158. Electrodes 168 may correspond to anyof the electrodes shown in FIG. 1. Switch matrix 158 may be used forselecting which electrodes and corresponding polarities are used fordelivering electrical stimulation pulses.

Cardiac electrical signals are sensed for use in determining when anelectrical stimulation therapy is needed and in controlling astimulation mode and the timing of stimulation pulses. Electrodes 168used for sensing are coupled to signal processing circuitry 160. Signalprocessor 160 includes sense amplifiers and may include other signalconditioning circuitry and an analog-to-digital converter. Electricalsignals may then be used by microprocessor 154 or other controlcircuitry for detecting physiological events, such as detecting anddiscriminating cardiac arrhythmias or detecting the need for pacing.Signal processing circuitry 160 may include event detection circuitrygenerally corresponding to P-wave detection and R-wave detectioncircuitry.

IMD 14 may optionally be coupled to one or more physiological sensors170, shown schematically. Physiological sensors 170 may include apressure sensor, a motion sensor or accelerometer, a flow sensor, bloodchemistry sensors such as an oxygen saturation sensor, activity sensors,an acoustical sensor, or other physiological sensors. Physiologicalsensors may be carried by any lead extending from IMD 14, incorporatedin or on the IMD housing or may be embodied as leadless sensorsimplanted in the body and in telemetric communication with the IMD oranother device. Signals from sensors 170 are received by a sensorinterface 162 which provides sensor signals to signal processingcircuitry 160. Sensor signals are used by microprocessor 154 fordetecting physiological events or conditions.

In various embodiments, IMD 14 includes rate responsive pacing in whichan activity sensor, oxygen sensor, respiration sensor, or other sensorgenerating a signal correlated to changes in metabolic demand, referredto generally as “demand sensor”, or any combination of demand sensors,is used to compute a sensor-indicated rate (SIR). Timing and controlmodule 152 responds to the SIR by adjusting an atrial pacing rate up ordown between a programmed lower rate and a maximum upper rate.

The IMD operating system includes associated memory 156 for storing avariety of operating algorithms and parameter values that are used bymicroprocessor 154. Algorithms and control parameters used forcontrolling minimum ventricular pacing, delivering rate responsivepacing, detecting arrhythmias, delivering arrhythmia therapy, monitoringevent intervals, and monitoring autonomic function may be stored inmemory 156. The memory 156 may also be used for storing data compiledfrom sensed EGM signals and/or relating to device operating history fortelemetry out on receipt of a retrieval or interrogation instruction.

IMD 14 further includes telemetry circuitry 164 and antenna 165.Programming commands or data are transmitted during uplink or downlinktelemetry between IMD telemetry circuitry 164 and external telemetrycircuitry included in a programmer or monitoring unit. Report module 174may compile data acquired by the IMD 14 for transmitting via telemetrycircuitry 164 to an external display 180, which may be implemented in aprogrammer, personal computer, web-based or local network, or othercommunication device in communication with IMD 14. In one embodiment,data relating to a percentage of time pacing in the atria and/orventricles and other heart rhythm information is compiled in a medicalreport that is transmitted to display 180 to be presented in a text orgraphical display to a clinician.

FIG. 3 is a timeline 200 depicting atrial and ventricular events sensedby a pacing device. The pacing device is sensing in both an atrium(events depicted on the upper portion of the time line 200) and aventricle (events depicted on the lower portion of time line 200). Thepacing device is operating in a MVP mode in which atrial-only pacing isin effect during intact atrial-ventricular conduction, as evidenced bythe intrinsic ventricular sensed (VS) events 206. Intrinsic atrialsensed (AS) events 202 are occurring at a rate faster than a programmedatrial pacing rate, causing atrial pacing pulses to be withheld. Inother words, an atrial pace escape interval timer restarted upon each ASevent 202 does not expire before the next AS event. An ARP 204 isstarted upon each AS event 202 such that any atrial events sensed duringthe ARP 204 are ignored for purposes of setting pacing timing intervals.

A VS event 208 is seen to occur following a previous VS event without anintervening AS event. The VS event 208 may be a premature ventricularcontraction (PVC) as indicated in the example of FIG. 3. In other cases,two consecutive VS events may occur without an intervening AS event whenan atrial P-wave is undersensed. Subsequent to the VS event 208, a PVARP210 is started. The next atrial event that is sensed occurs during PVARP210 resulting in an atrial refractory (AR) event 212. The AR event 212is ignored by the pacing device for the purposes of starting pacingtiming intervals. The next VS event occurs without a preceding AS eventand marks the start of another PVARP interval. As a result, the nextatrial event is again an AR event. This AR-VS pattern, which can beconversely referred to as a VS-AR pattern, may be sustained indefinitelyuntil a change in the heart rhythm occurs that results in either anatrial event outside PVARP 210, a missed or undersensed ventricularevent that prevents PVARP 210 from being set, or a mode switch to dualchamber pacing occurs due to a lack of intrinsic ventricular events.

FIG. 4 is an EGM recording 300 illustrating the delivery of a back-upventricular pacing pulse during atrial-only pacing. In panel A, asustained pattern of AR-VS sensing 301 is shown. The pacing device isoperating in an atrial-only pacing mode during MVP. Transientatrial-ventricular conduction block may occur resulting in ventricularasystole beginning on cardiac cycle 314. The last AR event 304 is notused by the pacing device in setting pacing intervals. As such, theAR-AS cycle 314 is not used to start a ventricular pace time interval.The next atrial event 308 is sensed outside of PVARP 306 since noventricular event occurred after AR event 304 to cause a PVARP to bereset. No ventricular event is sensed during the first AS-AS cycle 310.A ventricular back-up pacing interval 322 is therefore set upon sensingthe AS event 316 that is not preceded by a ventricular sensed eventduring AS-AS cycle 310. Upon the expiration of the ventricular paceinterval 322, a backup ventricular pace (VP) 320 is delivered. The AR-ASinterval 314 that occurs as a result of the sustained AR-VS sensingpattern effectively delays the delivery of a backup VP by one cardiaccycle.

In comparison, as shown in panel B, when intrinsic atrial andventricular events are properly sensed in an AS-VS pattern 351, the lastVS event 352 is followed by an AS event 354. When the next AS event 358occurs resulting in an AS-AS interval 360 without an intervening VSevent, the backup VP timing interval 372 is started. AVP 370 isdelivered upon expiration of timing interval 372 such that only a singleasystolic cardiac cycle 360 occurs. The VP 370 occurs earlier,approximately one cardiac cycle earlier, during accurate AS-VS sensing351, as compared to the result during an AR-VS sensing pattern 301 asshown in Panel A. Thus, it is desirable to interrupt a sustained AR-VSsensing pattern and restore a proper AS-VS pattern to promote a promptand appropriate response by the pacing device to changes in heartrhythm.

FIG. 5 is a flow chart 400 of a method for terminating an AR-VS sensingpattern during MVP. Flow chart 400 is intended to illustrate thefunctional operation of the device, and should not be construed asreflective of a specific form of software or hardware necessary topractice the methods described. It is believed that the particular formof software will be determined primarily by the particular systemarchitecture employed in the device and by the particular detection andtherapy delivery methodologies employed by the device. Providingsoftware to accomplish the described functionality in the context of anymodern IMD, given the disclosure herein, is within the abilities of oneof skill in the art.

Methods described in conjunction with flow charts presented herein maybe implemented in a computer-readable medium that includes instructionsfor causing a programmable processor to carry out the methods described.A “computer-readable medium” includes but is not limited to any volatileor non-volatile media, such as a RAM, ROM, CD-ROM, NVRAM, EEPROM, flashmemory, and the like. The instructions may be implemented as one or moresoftware modules, which may be executed by themselves or in combinationwith other software.

The process shown in the flow chart 400 is performed when the pacingdevice is operating in an atrial-only pacing mode during MVP asindicated at block 402. At block 404, atrial and ventricular sensedevents are analyzed to determine if a sustained AR-VS pattern isdetected. A sustained AR-VS pattern may be detected when a specifiednumber of consecutive AR events are detected. In one embodiment, the ARevents are counted and if the counter reaches a specified number, e.g.eight AR events, a sustained AR-VS pattern is detected. An AS eventwould reset the counter. The counts used to detect sustained atrialrefractory sensing may include counts of consecutive AR events, AR-VSsequences, or VS-AR sequences.

Upon detecting the sustained AR-VS pattern, the PVARP set in response tothe next VS event is set at block 406 to an interval shorter than aninitial PVARP being set during the sustained AR-VS sensing pattern. Theshortened PVARP may be a predetermined interval less than the initialPVARP or may even be set to 0 ms.

FIG. 6 is a time line 500 of a sustained AR-VS pattern 502 terminated bysetting a single, shortened PVARP 512. A sustained AR-VS pattern 502 isdetected upon sensing the nth consecutive AR event 508, where n is apreviously specified number of consecutive AR events defined as criteriafor detecting a sustained AR-VS sensing pattern. In response todetecting the sustained AR-VS pattern, the PVARP 512 is set followingthe nth VS event 510 to a shortened interval relative to the PVARP 504applied during the AR-VS pattern 502. The shortened PVARP 512 will causethe next atrial event 514 to be a non-refractory event sensed outsidethe PVARP thereby terminating the AR-VS pattern and establishing anAS-VS sensing pattern 520. The AS-VS pattern 520 is established and theatrial-only pacing mode of the pacing device is maintained.

The shortened PVARP 512 may be set to a previously specified interval ora specified decrement less than the initial PVARP 504. In someembodiments, a VS-AR interval 506 is measured during the sustained AR-VSpattern 502. The shortened PVARP 512 is set to be shorter than themeasured VS-AR interval 506 to promote sensing of the next atrial event514 outside the shortened PVARP 512. For example, the shortened PVARP512 may be set to approximately 50 ms, or a larger decrement, less thanthe measured VS-AR interval 506. The VS-AR interval measurement used toset the shortened PVARP 512 may be a single interval measurement or aminimum, average or other measure of multiple VS-AR intervals.

Referring again to flow chart 400 in FIG. 5, the process may optionallymonitor for a VS-AR-AS pattern at block 408 during atrial-only pacing.If a VS-AR-AS pattern occurs, the AR-AS cycle may represent a cycle ofventricular asystole. In this case, the backup ventricular pace intervalmay be started at block 410 upon sensing the AS event of a VS-AR-ASpattern so that delivery of a ventricular backup pace is not delayed bythe AR event. In other words, the AR-AS sequence occurring without anintervening VS event is used in setting a VP timing interval rather thanwaiting for an AS-AS interval. Additional criteria may be applied atblock 410 for detecting a VS-AR-AS pattern that may be associated withventricular asystole. For example, a preceding AR event may be requiredsuch that the pattern AR-VS-AR-AS is detected at block 408. Otherrequirements may include a PVC detected prior to the VS-AR-AS pattern,i.e. a VS-VS cycle detected without an intervening atrial event. TheVS-VS cycle may be the immediately preceding cycle leading into theVS-AR-AS pattern (i.e., VS-VS-AR-AS) or a VS-VS cycle may be required tohave occurred within a preceding number of cardiac cycles prior to theVS-AR-AS pattern with all intervening atrial events being AR events.Such a pattern can be represented by VS-VS-AR-(VS-AR)_(i)-AS where i maybe any number that is less than the specified number of consecutive ARevents required to detect a sustained AR-VS pattern. Evidence of aprevious VS-VS cycle or a preceding AR-VS interval leading up to thedetected VS-AR-AS pattern suggests that the atrial refractory eventoccurring in the VS-AR-AS pattern may actually be a normal atrialdepolarization and may be appropriately used in setting a backup VPtiming interval.

If the backup VP interval expires as determined at block 412, aventricular pacing pulse is delivered at block 414 and the processreturns to block 402. The pacing device remains in the atrial-onlypacing mode. If the VP interval does not time out, i.e. a VS event issensed prior to expiration of the VP interval, the process returns toblock 402 without delivering the ventricular pacing pulse.

Thus, a pacing device and associated method have been presented in theforegoing description with reference to specific embodiments. It isappreciated that various modifications to the referenced embodiments maybe made without departing from the scope of the invention as set forthin the following claims.

The invention claimed is:
 1. A method for controlling intervals timed byan implantable cardiac pacing device configured for delivering cardiacpacing, the method comprising: sensing intrinsic atrial events in anatrial chamber; sensing intrinsic ventricular events in a ventricularchamber; setting a first post-ventricular atrial refractory periodfollowing each of a plurality of ventricular events sensed in theventricular chamber; detecting a plurality of atrial events each beingsensed during first post-ventricular atrial refractory periods; andsetting a second post-ventricular atrial refractory period following asingle intrinsic ventricular event in response to detecting theplurality of atrial events, the second post-ventricular atrialrefractory period being shorter than the first post-ventricular atrialrefractory period so that a next intrinsic atrial event is sensedoutside the second post-ventricular atrial refractory period andestablishes a pattern of an atrial non-refractory event sensed in theatrial chamber (AS) followed by a ventricular event sensed in theventricular chamber (VS).
 2. The method of claim 1 wherein setting thesecond post-ventricular atrial refractory period comprises: measuring aninterval between a sensed ventricular event and an atrial event sensedduring the first post-ventricular atrial refractory period; and settingthe second post-ventricular atrial refractory period to be less than themeasured interval.
 3. The method of claim 1 wherein the secondpost-ventricular atrial refractory period is set to zero ms.
 4. Themethod of claim 1 wherein detecting the plurality of atrial eventscomprises detecting a predetermined number of consecutive atrial eventseach sensed during consecutive first post-ventricular atrial refractoryperiods.
 5. The method of claim 1 further comprising: detecting aVS-AR-AS pattern upon sensing an atrial event within the first postventricular atrial refractory period and sensing a subsequent intrinsicatrial event outside a first post ventricular atrial refractory periodwithout sensing an intervening ventricular event; setting a ventricularpacing escape interval in response to detecting the VS-AR-AS pattern;and delivering a ventricular pace upon expiration of the pacing escapeinterval.
 6. The method of claim 5 further comprising sensing at leastone atrial event during a first post ventricular atrial refractoryperiod preceding the detected VS-AR-AS pattern prior to setting theventricular pacing escape interval.
 7. An implantable medical deviceconfigured to control timing intervals used by the device for sensingcardiac activity and delivering cardiac pacing pulses, the devicecomprising: an atrial pacing and sensing lead for delivering atrialpacing pulses and sensing atrial signals; a ventricular pacing andsensing lead for delivering ventricular pacing pulses and sensingventricular signals; a sensing module coupled to the atrial andventricular pacing and sensing leads, the sensing module responsive tointrinsic depolarization events of a heart; a pulse generator producingcardiac stimulation pulses delivered via the atrial and ventricularpacing and sensing leads; and a control module coupled to the sensingmodule and the therapy delivery module, the control module configuredto: set a first post-ventricular atrial refractory period following eachof a plurality of ventricular events sensed in the ventricular chamber;detect a plurality of atrial events each being sensed during firstpost-ventricular atrial refractory periods; set a secondpost-ventricular atrial refractory period following a single sensedventricular event in response to detecting the plurality of atrialevents, the second post-ventricular atrial refractory period beingshorter than the first post-ventricular atrial refractory period so thata next intrinsic atrial event is sensed outside the secondpost-ventricular atrial refractory period and establishes a pattern ofan atrial non-refractory event sensed in the atrial chamber (AS)followed by a ventricular event sensed in the ventricular chamber (VS).8. The device of claim 7 wherein the control module is furtherconfigured to: measure an interval between a sensed ventricular eventand an atrial event sensed during the first post-ventricular atrialrefractory period; and set the second post-ventricular atrial refractoryperiod to be less than the measured interval.
 9. The device of claim 7wherein the control module is configured to set the secondpost-ventricular atrial refractory period is set to zero ms.
 10. Thedevice of claim 7 wherein the control module is configured to detect theplurality of atrial events by detecting predetermined number ofconsecutive atrial events each sensed during consecutive firstpost-ventricular atrial refractory periods.
 11. The device of claim 7wherein the control module is further configured to: detect a VS-AR-ASpattern upon sensing an atrial event within the first post ventricularatrial refractory period and sensing a subsequent intrinsic atrial eventoutside a first post ventricular atrial refractory period withoutsensing an intervening ventricular event; set a ventricular pacingescape interval in response to the VS-AR-AS pattern; and deliver aventricular pacing pulse upon expiration of the pacing escape interval.12. The device of claim 11 wherein the control module is furtherconfigured to sense at least one atrial event during a first postventricular atrial refractory period preceding the detected VS-AR-ASpattern prior to setting the ventricular pacing escape interval.
 13. Acomputer-readable medium storing a set of instructions which whenimplemented in an implantable medical device configured to controltiming intervals used by the device for sensing cardiac activity anddeliver cardiac pacing pulses and cause the device to: sense intrinsicatrial events in an atrial chamber; sense intrinsic ventricular eventsin a ventricular chamber; set a first post-ventricular atrial refractoryperiod following each of a plurality of ventricular events sensed in theventricular chamber; detect a plurality of atrial events each beingsensed during first post-ventricular atrial refractory periods; and seta second post-ventricular atrial refractory period following a singleintrinsic ventricular event in response to detecting the plurality ofatrial events, the second post-ventricular atrial refractory periodbeing shorter than the first post-ventricular atrial refractory periodso that a next intrinsic atrial event is sensed outside the secondpost-ventricular atrial refractory period and establishes a pattern ofan atrial non-refractory event sensed in the atrial chamber (AS)followed by a ventricular event sensed in the ventricular chamber (VS).14. The computer-readable medium of claim 13 further comprisinginstructions which cause the device to: measure an interval between asensed ventricular event and an atrial event sensed during the firstpost-ventricular atrial refractory period; and set the secondpost-ventricular atrial refractory period to be less than the measuredinterval.
 15. The computer-readable medium of claim 13 furthercomprising instructions which cause the device to set the secondpost-ventricular atrial refractory period is set to zero ms.
 16. Thecomputer-readable medium of claim 13 further comprising instructionswhich cause the device to detect the plurality of atrial events bydetecting predetermined number of consecutive atrial events each sensedduring consecutive first post-ventricular atrial refractory periods. 17.The computer-readable medium of claim 13 further comprising instructionswhich cause the device to: detect a VS-AR-AS pattern upon sensing anatrial event within the first post ventricular atrial refractory periodand sensing a subsequent intrinsic atrial event outside a first postventricular atrial refractory period without sensing an interveningventricular event; set a ventricular pacing escape interval in responseto the VS-AR-AS pattern; and deliver a ventricular pacing pulse uponexpiration of the pacing escape interval.
 18. The computer-readablemedium of claim 17 further comprising instructions which cause thedevice to sense at least one atrial event during a first postventricular atrial refractory period preceding the detected VS-AR-ASpattern prior to setting the ventricular pacing escape interval.